1. Field of the Invention
The present invention relates to a positive-type photosensitive polyimide precursor and a composition comprising the same. More specifically, the present invention relates to a polyamic ester having one or more reactive end-capping groups at either or both terminals of its molecular chain, wherein the ester structure of the polyamic ester degrades to yield carboxylic acid upon generation of acid by exposure; a mixture of at least one polyamic ester and at least one polyamic acid, the polyamic acid also having one or more reactive end-capping groups at either or both terminals of its molecular chain; and a composition comprising the mixture for use as a photosensitive polyimide precursor material.
2. Description of Background Art
In the field of semiconductor devices, and especially in the areas of semiconductor memory devices and liquid crystal display (LCD) devices, much effort has been made to date to improve the level of integration, densification, integrity, reliability and speed of semiconductor devices. In this connection, many advantages of organic materials, such as facility of processing and purification, are noteworthy. However, only those organic materials that are thermally stable at a temperature of 200xc2x0 C. or higher may be used in this field.
Polyimide resins were approved suitable for the purposes as described above because of the following reasons: Polyimide resins exhibit excellent thermal resistance and mechanical strength while possessing excellent electrical properties by virtue of having a low dielectric constant and a high insulation capacity. Polyimide resins also provide coating films that exhibit excellent planation properties. The low level of impurities in polyimide resins also helps to increase the integrity and reliability of final semiconductor devices. From an application perspective, polyimide resins are easy to process in forming fine patterns.
Generally, polyimide resins are produced by a two-step condensation polymerization method. First, diamine and dianhydride are subjected to polymerization in a polar solvent such as NMP, DMAc or DMF to provide a polyimide precursor solution. Second, the polyimide precursor solution is coated onto a silicon wafer or a glass substrate and then cured through heat treatment. Commercial polyimide products for use in electronic industry are supplied as a polyimide precursor solution or as a polyimide film. In the field of semiconductor devices, polyimide precursor solutions are commonly used.
FIG. 1 shows a sectional structure of a semiconductor device, wherein a polyimide resin is applied in a buffer coating film of the device. Plastic packages of large scale integration (LSI) are subject to various physical forces, including contraction after a packaging process as well as thermal stress due to differences in coefficients of thermal expansion between a chip and the resin. These physical forces result in either cracks in a passivation film or damage to metal lines or both. In order to alleviate these types of problems, a buffer layer is formed between the chip and the package using a polyimide film. In order to obtain a buffer effect, the thickness of the polyimide film buffer layer should be at least 10 xcexcm. Generally, thicker polyimide films result in increased buffer effects and helps to increase final production yield of semiconductor products. As shown in FIG. 1, fine patterns should be formed in the polyimide film, such as interconnection of electrodes and wire bonding pads. Typically, via holes in a polyimide film are formed by coating a conventional photoresist onto the polyimide film and etching the photoresist film. Recently, several photosensitive polyimides have been proposed, which were prepared by modifying polyimides to have inherent photosensitivity.
When conventional non-photosensitive polyimides are used in the buffer coating film, a separate etching process is required, and via holes for wire bonding and connections between metal lines are formed by using a photoresist. When photosensitive polyimides are employed, the use of a photoresist can be omitted. The elimination of the need for the photoresist reduces the overall buffer coating process by about 50% and results in higher productivity and lower production costs. Additionally, the final steps in the assembling process is reduced, which in turn further contributes to enhancing final production yield. Based on these advantages, research on photosensitive polyimides has been actively pursued.
The first practical photosensitive polyimide was developed by Rubner et al. on behalf of Siemens AG, Germany (U.S. Pat. No. 3,957,512), wherein photosensitive groups are attached to a polyimide precursor, i.e., polyamic acid via ester bonds. According to this U.S. patent, a polyimide precursor solution is coated onto a substrate to form a film, and the film is exposed to UV light so that photopolymerization can occur in the exposed region and form cross-linkages between the precursor molecules. The film is then subjected to development using an organic solvent to remove unexposed regions, followed by thermal treatment. During the thermal treatment, an imidization reaction occurs, and the ester-bonded photosensitive groups degrade to provide a desired pattern made of polyimide.
U.S. Pat. No. 4,243,743 assigned to Toray Co., Ltd., Japan, proposes a photosensitive polyimide, wherein photosensitive groups and compounds with an amino group are attached to a polyamic acid via ion bonds. Such a photosensitive polyimide is advantageous over conventional photosensitive polyimides from the standpoint of ease in preparation and relatively fewer toxic side products.
Currently, photosensitive polyimides are preferred over negative-type photosensitive polyimides, because positive-type photosensitive polyimides exhibit superior resolution. Moreover, positive-type photosensitive polyimides have a relatively small area of exposure and, consequently, are associated with a lower frequency of inferior products. Additionally, alkaline solutions are used as a developer for positive-type photosensitive polyimides, and since alkaline solutions to not generate environmental pollution issues, they enable reduction of processing costs. Negative-type photosensitive polyimides, on the other hand, use toxic organic solvents such as NMP and DMAc as developers and are problematic considering the cost factor and the environmental pollution caused by the liquid waste products. Production of positive-type photosensitive polyimides, however, has not been truly commercialized to date because of a large difficulty that must be overcome.
In prior art teachings relative to positive-type photosensitive polyimides, Japanese Laid-Open Publication Nos. 52-13315 and 62-135824 disclose a patterning method whereby a pattern is formed by virtue of the different dissolution rate of exposed and unexposed regions while using a mixture of polyamic acid as a polyimide precursor and naphthoquinonediazide as a dissolution inhibitor. Japanese Laid-Open Publication No. 64-60630 discloses a method for patterning using a mixture of soluble polyimide having hydroxyl groups and naphthoquinonediazide. Japanese Laid-Open Publication No. 60-37550 discloses a method for patterning by using a photosensitive polyimide, wherein the photosensitive polyimide was prepared through connecting an o-nitrobenzylester group as a photosensitive group to a polyimide precursor via an ester bond. These prior art teachings, however, are not satisfactory because of the following disadvantages. Firstly, the difference in dissolution rate between exposed regions and unexposed regions is not sufficient to form a pattern having high resolution (See: Japanese Laid-Open Publication Nos. 52-13315 and 62-135824). Secondly, the prior polyimide precursors are limited in structure and are poor in transparency and other physical properties (See: Japanese Laid-Open Publication No. 60-37550). Thirdly, the relatively low sensitivity makes an increase of film thickness difficult (See: Japanese Laid-Open Publication No. 60-37550). Fourthly, a photosensitizer with high absorbance is required in large amounts, and as a result, the transparency of final compositions decreases and makes it difficult to form a pattern of high resolution.
Alternatively, a chemical amplification-type resin composition was recently proposed by Nitto Denko Co., Ltd, Japan, which chemical amplification-type resin composition was prepared by mixing a resin and a photo acid generator. The resin was obtained from substitution of carboxylic groups in polyamic acid with acetal groups that can be dissociated by acid. (See: Japanese Laid-Open Publication Nos. 7-33874 and 7-134414). A patterned film made from the composition was shown to have good residual percentage of film, but poor post-curing shrinkage and elongation of film because of a low degree of imidization of the acetal-substituted polyamic acid. Typically, in order to achieve excellent processing properties during a patterning process, a resin composition should contain as much solid resin as possible while maintaining low viscosity. If, however, the molecular weight of the resin is reduced for this purpose, deterioration of physical properties of final films, such as elongation, became more significant. This result makes it difficult to make common use prior art resins.
A feature of an embodiment of the present invention is to provide a polyamic ester with a novel structure, affording a film with excellent elongation, wherein acid labile groups are introduced into carboxylic groups of a polyamic acid to yield the polyamic ester and one or more reactive end capping monomers are added to either or both terminals of the molecular chain of the polyamic ester.
Another feature of an embodiment of the present invention is a mixture of polyimide precursors comprising at least one polyamic ester and at least one polyamic acid, wherein reactive end-capping groups that can cross-link each other upon thermal curing were added to either or both terminals of respective molecular chains of the polyamic ester and the polyamic acid. In doing so, the molecular weight of the polymers before curing can be lowered enough to optimize the balance of solid content and viscosity of a final resin composition, while obtaining a film of high quality through curing that exhibits excellent properties including minimal film shrinkage.
Yet another feature of the present invention is a photosensitive polyimide precursor composition comprising the mixture of polyimide precursors that can form films of high quality and excellent elongation.
According to an embodiment of the present invention, there is provided a polyamic ester prepared by partially substituting hydrogen atoms of carboxylic groups of a polyamic acid with acid labile groups, wherein the polyamic ester comprises one or more repeating units represented by the Formula 1, and each of at least one terminal of the polyamic ester molecule terminates with the same or different reactive end-capping monomer: 
In Formula 1,
R1 and R2 are independently a hydrogen atom or an acid labile group;
X is a tetravalent, an aromatic or an aliphatic organic group;
Y is a divalent, an aromatic or an aliphatic organic group; and
m is an integer equal to or greater than 1.
Accordingly to another embodiment of the present invention, there is provided a polyimide precursor mixture comprising at least one polyamic ester and at least one polyamic acid, wherein the polyamic acid comprises one or more repeating units represented by the following Formula 6, wherein each of at least one terminal of the polyamic acid molecule terminates with the same or different reactive end-capping monomer: 
In Formula 6,
X is a tetravalent, an aromatic or an aliphatic organic group;
Y is a divalent, an aromatic or an aliphatic organic group; and
n is an integer equal to or greater than 1.
According to another embodiment of the present invention, there is provided a photosensitive polyimide precursor composition comprising (a) the polyimide precursor mixture, (b) one or more photo acid generators and (c) one or more polar solvents.
These and other features and aspects of the present invention will be readily apparent to those of ordinary skill in the art upon review of the detailed description that follows.